1. Field of the Invention
The present invention relates to duplex printing transfer paper used in indirect dry electrophotographic full-color and monochrome copying machines and printers.
2. Description of the Related Art
In an image forming apparatus such as an electrophotographic copying machine, generally, a transfer method for transferring a toner image onto transfer paper supported on a transfer drum while a transfer material such as transfer paper is being supported on a transfer material holding member such as the transfer drum driven to rotate in synchronism with a toner image carrier such as a photosensitive material drum is mainly used in a color copying machine. In the method, a plurality of toner images can be transferred one over another with good accuracy because the toner images are piled by transferring the toner images successively onto transfer paper supported on the transfer drum.
On the other hand, in a so-called tandem type color copying machine, an endless belt-shaped transfer material holding member is used instead of the aforementioned transfer drum, and a plurality of image forming means corresponding to respective colors are arranged along the movement direction of the transfer material holding member. A color image is formed by transferring respective color toner images successively formed by the respective image forming means onto a transfer material held on the transfer material holding member. The tandem type color copying machine is generally expensive because such a plurality of image forming means are required. However, the copying machine has an advantage in improvement of copying efficiency because a multicolor toner image can be transferred and formed onto a transfer material in a period in which the transfer material is carried by the endless belt-like transfer material holding member.
There is a further method in which a toner image on a photosensitive material is once primarily transferred onto an intermediate transfer material other than paper and then the toner image is secondarily transferred onto paper to obtain a copy image. This method has an effect that failure of multiple transferring and displacement of color registration caused by a lot of factors such as paper holding condition, paper thickness, paper stiffness, and surface property, can be suppressed from occurring in a color copying machine performing multiple transferring.
FIG. 1 is a general structural diagram of a color copying machine using a conventional transfer drum, which comprises an automatic document supply unit 1, an image input portion 2, an image output portion 3, and a paper supply portion 4. Copy cycle in the case of a full color mode will be described below. A color document is set on platen glass 5 by the automatic document supply unit 1. The image input portion 2 includes an imaging unit 6, and a wire 7, a drive pulley 9, and the like for driving the unit. In the case of four colors in full, the image input portion 2 reads the color document by B (blue), G (green) and R (red) as primary colors of light and converts the color document into a digital image signal by using a CCD line sensor and a color filter disposed in the imaging unit 6. Then, the image input portion 2 converts this signal into Y (yellow), C (cyan), M (magenta) and K (black) as primary colors of toner and further converts the color gradation toner signal into an on/off two-valued signal by applying various data processing in order to improve reproducibility in color, gradation, definition, and the like Thus, the image input portion 2 outputs the two-valued signal to the image output portion 3.
The image output portion 3 includes a scanner 10 and a photosensitive material drum 11. Further, there are arranged an electrifier 12 for electrifying the photosensitive material drum 11 uniformly, a developer unit 13 for developing an electrostatic latent image to a toner image, a transfer drum 16 for transferring the toner image onto paper, and a cleaner 15 for recovering the residual toner which has not transferred. The photosensitive material drum 11 is driven by an electric motor so as to rotate in the direction of the arrow shown in the drawing.
In a laser output portion 10a of the scanner 10, for example, a yellow image signal from the aforementioned image input portion 2 is converted into a light signal so that a latent image corresponding to the document image is formed on the photosensitive material drum 11 through a polygon mirror 10b, an f/.theta. lens 10c and a reflection lens 10d. If this yellow latent image is transferred onto paper through development, the residual toner is removed from the photosensitive material drum 11 by the cleaner 15 and then the photosensitive material drum 11 is electrified by the electrifier 12 so that the laser output portion 10a outputs a cyan image signal. Thereafter, latent images of magenta and black image signals are formed successively.
The developer unit 13 has a yellow developer 13Y, a cyan developer 13C, a magenta developer 13M, and a black developer 13K. The respective developers are arranged around a rotary shaft. When, for example, a yellow toner image is to be formed, development is performed by the yellow developer 13Y in the position shown in the drawing. When, for example, a cyan toner image is to be formed, the development unit is rotated so that the cyan developer 13C is arranged in a position where the cyan developer 13C touches the photosensitive material drum 11. Magenta and black developments are carried out in the same manner as described above.
A dielectric film or a mesh screen is put up in the outer periphery of the transfer drum 16. The transfer drum 16 is connected to an exclusive-use electric motor or the photosensitive drum 11 by a gear so that the transfer drum 16 is driven to rotate in the direction of the arrow shown in the drawing. A transfer electrifier 17, a separation electrifier 19, a peel claw 20, a destaticizer 21, a cleaner 22, a push roll 23 and an adsorption electrifier 25 are arranged in the periphery of the transfer drum 16. Paper carried from the paper supply portion 4 via paper supply rollers 26 and paper supply guides 27 is held on the dielectric film or mesh screen by corona of the adsorption electrifier 25. The transfer drum 16 rotates in synchronism with the photosensitive material drum 11, so that, for example, a toner image developed by yellow is transferred onto the paper by the transfer electrifier 17 and other colors are transferred successively by the rotation of the transfer drum 16.
When transfer of four colors is completed by four turns of the transfer drum, the transfer drum 16 is AC-destaticized by the separation electrifier 19 provided on the transfer drum 16, so that the paper is separated by the peel claw 20 and fed to a fixer 30 by a carrying belt 29. The toner image is melted and fixed by hot-press rollers 31. Thus, a-copying cycle is completed.
As shown in FIG. 2, the transfer drum 16 is constituted by a body 16e and a transfer film 16a fixed to the body 16e so that the transfer drum 16 is shaped like a hollow cylinder. In the body 16e, cylindrical members 16b and 16c are united at opposite sides with a tie-bar 16d connecting these cylindrical members 16b and 16c, for example, by alminium die casting.
If conventional electrophotographic transfer paper is used particularly in a copying machine or printer of an indirect dry electrophotographic method using such a transfer drum selected from the aforementioned transfer methods, toner is not transferred in the peripheral portion of the second surface particularly in the case where a full-color image is formed on the whole surface of the transfer paper in the same manner as described above at a relatively high humidity and then a whole-surface full-color or monochrome image is formed again on a surface opposite to the image forming surface, that is, in the case where duplex copying is performed. As a result, the image is missing. There arises a problem that partial deletion occurs so as to form white partial deletion portions shaped like semicircles, fingers, and the like as shown in FIG. 3. Further, besides the aforementioned full-color transfer method, the same problem arises in a method for transferring multicolor toners collectively onto transfer paper.
The aforementioned partial deletion is a phenomenon which is newly recognized because there has become frequent the case where a document having an image on its whole surface as often seen in a photographic document, and the like is copied onto opposite surfaces of conventional electrophotographic transfer paper by a conventional color copying machine. Therefore, the phenomenon was not recognized when a document having a low-density image without any image in its peripheral portion as often seen in a character document, and the like had been copied onto opposite surfaces of conventional electrophotographic transfer paper by a monochrome copying machine.
In duplex copying, after a high density image such as a whole-surface full-color image is formed on the first surface of conventional electrophotographic transfer paper, this image formed surface is held so as to contact with a transfer material holding member such as a transfer drum or an endless belt-like transfer material holding member. At this time, as shown in FIGS. 4 and 5, a gap exists because the peripheral portion of transfer paper is not perfectly brought into contact with the transfer material holding member. Consequently, when an image on the second surface exists in the whole surface whether full-color or monochrome, the partial deletion phenomenon occurs in the peripheral portion of the second surface in duplex copying. Here, even in the case of a low-density image, partial deletion is recognized so long as the image on the second surface exists in the whole surface. Of course, when the aforementioned image on the second surface does not exist in the whole surface, for example, in the case of a character image, or the like, having blanks in the peripheral portion, partial deletion does not occur. Exactly, because there is originally no image in the peripheral portion of the second surface of transfer paper, there is no toner image transferred thereto, that is, the peripheral portion is left as a blank space, so that partial deletion cannot be recognized.
It is confirmed that the aforementioned gap is caused by the fact that the surface wariness of transfer paper or the curling of transfer paper toward the image side increases particularly when a full-color image is formed on the whole surface of the first surface of transfer paper in duplex copying. The curling toward the image side is caused by the effect that the contracting force of the toner layer overcomes the bending stiffness of transfer paper, that is, bimetal effect. Accordingly, a full-color image formed from four color toner layers of black, yellow, magenta and cyan has a thicker toner layer than a black-white or monochrome image formed from only one monochrome toner layer, that is, the contracting force of the toner layer increases so that the curling becomes large.
Further, it is confirmed that the aforementioned gap is formed easily when the axis of the curling is formed in the direction of feeding of transfer paper on the basis of balance among fiber orientation aspect ratio, stiffness in MD (Machine Direction, wherein the paper direction is parallel to the direction of movement of the paper), stiffness in CD (Cross Direction, wherein the paper direction is perpendicular to the direction of movement of the paper machine), the quantity of contraction of the toner layer, and the like, because particularly in the case where the transfer material holding member is a transfer drum, the curling is turned to a direction which is not along the curvature of the transfer drum as shown in FIG. 6. Contrariwise, it is confirmed that the aforementioned gap is not formed when the axis of the curling is formed in a direction perpendicular to the direction of feeding of transfer paper, because particularly in the case where the transfer material holding member is a transfer drum, the curling is turned to a direction which is along the curvature of the transfer drum as shown in FIG. 7.
Further, it is confirmed that a gap is formed in the peripheral portion of transfer paper regardless of the kind of the transfer material holding member when the aforementioned surface wariness is increased because of the contraction of fiber caused by dehumidification of transfer paper at the time of hot-press fixing, and the expansion of fiber caused by humidification of transfer paper after the hot-press fixing correspondingly to the fiber orientation ratio of transfer paper, the characteristic of pulp, and the like. It is confirmed that the gap is formed more easily when the condition of higher humidity is given as the environment in which the surface waviness is increased because of humidification/dehumidification of transfer paper.
Incidentally, when the deformation of transfer paper is relatively small though transfer paper is electrostatically held from its front end by the transfer material holding member with respect to the direction of feeding of transfer paper, the deformation can be escaped on the way of this holding. Accordingly, the aforementioned gap is not formed and partial deletion does not occur. When the deformation of transfer paper is contrariwise very large particularly in its rear end, the deformation cannot be escaped on the way of the holding. Accordingly, the aforementioned gap is formed and it is confirmed that partial deletion occurs easily particularly in the rear end portion of the second surface of transfer paper.
It is apparent from the aforementioned partial deletion phenomenon that the partial deletion used herein is a kind of transfer failure which occurs because the transfer condition is changed by a gap formed between the transfer paper and the transfer material holding member on the basis of the physical deformation of transfer paper due to the surface wariness or curling at the time of duplex copying. Transfer failure known conventionally is, however, clearly different from partial deletion at the time of duplex copying, because the conventionally known transfer failure is a transfer failure at the time of simplex copying in a phenomenon that a transfer image having density lowered as a whole is formed or a transfer image is spotted because of scattering of toner under the environment of high humidity or low humidity.
In most cases, the transfer failure in conventional electrophotographic transfer paper occurs because the resistivity of the transfer paper under the environment of high humidity or low humidity is out of proper range for transfer. To improve such transfer failures, there are proposals in which a specific material is used for controlling the resistivity of transfer paper to be in a certain proper range. For example, proposals for plain paper type transfer paper are made in Unexamined Japanese Patent Publication (kokai) Nos. Hei-3-186855 and Hei-5-53363.
Proposals for coating paper type transfer paper are made in Unexamined Japanese Patent Publication Nos. Sho-62-198877 and Hei-3-242654. Further, a proposal for conventional coating type transfer paper is made in Unexamined Japanese Patent Publication No. Hei-5-297621 in which transfer failure caused by discharge irregularity is improved by controlling the diameter of a gap in the original paper layer to be in a proper range.
Conventionally, there are proposals for controlling the curl of electrophotographic transfer paper (Unexamined Japanese Patent Publication Nos. Hei-6-110243, Hei-6-138688 and Hei-6-194860) in which the curl or distortion after hot-press fixing is reduced in order to improve runnability and tray storing characteristic in a monochrome copying machine. They depend on means of setting the separation freeness difference between the front and rear layers of transfer paper to be not larger than a predetermined value, means of setting the fiber orientation index difference between the front and rear layers to be in a predetermined range, means of setting the angle of fiber orientation with respect to MD to be not larger than 10 degrees, and the like. These means do not form means of controlling the curl particularly after hot-press fixing of a full-color image in which four color toner layers are formed. There is much less consideration upon the problem of partial deletion at the time of duplex full-color copying and upon means of controlling the curl or surface wariness of transfer paper as means for improving the problem.
There is a further proposal for controlling the curl (Unexamined Japanese Patent Publication No. Hei-5-341554) in which the curl after hot-press fixing, the curl at the time of humidification and the curl difference between the front and rear surfaces are reduced in order to improve the storage capacity of a sorter or tray, the paper choking therein, and the like, taking into account duplex copying in a monochrome copying machine. The proposal depends on means of setting the CD contracting percentage of transfer paper to be not larger than 0.45% and setting the contracting percentage difference between the front and rear surfaces to be in a range of .+-.0.02%. There is no consideration upon the problem of partial deletion at the time of full-color duplex copying and upon means of controlling the curl or surface waviness of transfer paper as means for improving the problem.
Conventionally, there is a further proposal for controlling the curl of a full-color image on electrophotographic transfer paper (Unexamined Japanese Patent Publication No. Hei-5-341553) in which the curl after hot-press fixing is reduced by setting the CD moisture expansivity to be not larger than 0.45% and satisfying the relation Et.sup.3 .gtoreq.0.26, where E represents elastic modules in CD tension, and t represents paper thickness. There is, however, no consideration upon the aforementioned problem of partial deletion at the time of duplex copying, so that the proposal is insufficient to provide means of controlling the curl or surface wariness of transfer paper as means for improving the problem. Further, if the CD of transfer paper is a feeding direction perpendicular to the direction of feeding of paper, the problem of partial deletion at the time of duplex copying arises when Et.sup.3 is in a range of from about 0.26 to about 0.33 even in the case where the CD moisture expansivity is not larger than 0.45%. Further, even in the case where the CD moisture expansivity and Et.sup.a are controlled simply, the problem of partial deletion at the time of duplex copying may arise correspondingly to the relation between the MD moisture expansivity and Et.sup.3 at that time when the MD of transfer paper is a feeding direction perpendicular to the direction of feeding of paper.
There is a further proposal for duplex recordable full-color electrophotographic transfer paper (Japanese Patent Unexamined Publication No. Hei-6-18669) in which nonshowthrough is improved by a generally known method in which titanium dioxide, or the like, is used as a filler or pigment in order to set brightness and opacity to be not lower than predetermined values respectively. There is, however, no consideration upon the problem of partial deletion at the time of full-color duplex copying and upon means of controlling the curl or surface wariness of transfer paper as means for improving the problem.